1. Field of Invention
The present invention is related to a molybdenum disilicide (MoSi.sub.2) based ceramic composite heating element, more particularly to the heating element of a heat-treatment furnace, in which severe temperature-distribution is required, such as an oxidation, diffusion or low-pressure chemical-vapor-deposition (LP-CVD) furnace for producing semiconductors. In addition, the present invention is related to a production method of the molybdenum-disilicide based ceramic composite heating element.
2. Description of Related Art
Heretofore, an Fe--Cr--Al based, metallic heating element has been used in a heat-treatment furnace for producing semi-conductors, such as an oxidizing, diffusion or LP-CVD furnace. In a recent development of the rapid thermal processing, which pursues micro-devices and enhances cost competence in the production of semiconductor devices, the MoSi.sub.2 heating element has been used. That is, since the heat resistance of the MoSi.sub.2 heating element is superior to that of a metallic heating element, the MoSi2 heating element can be used at a surface power density as high as approximately ten times that of the metallic heating element. For example, the permissible level of the surface powder density of the metallic heating element at 1000.degree. C. is around 2 W/cm.sup.2, while that of the MoSi.sub.2 heating element at 1000.degree. C. is 20 W/cm.sup.2. Furthermore, by means of using the molybdenum-disilicide heating element and omitting a liner tube, which is usually made of silicon carbide and installed inside the heating element and the heat capacity of the heat treatment furnace is dramatically decreased. Therefore, only an inner tube made of quartz is installed inside the heating element as a processing tube. This feature readily leads to increasing the temperature-elevation and lowering rates. Specifically, in the case of a diffusion furnace, one heat-treatment time can be shortened by 60% or more, i.e., the conventional 200 minutes or more can be shortened to 80 minutes or less.
When the MoSi.sub.2 heating element is operated in a temperature of 1400.degree. C. or more and under ambient atmosphere, since a dense protective film consisting of silica (SiO.sub.2) is formed on the surface of the heating element, the heating element can be stably operated. However, in a temperature range of 400 to 1200.degree. C., where the oxidation, diffusion or LP-CVD furnace is operated, low-temperature oxidation, which is a characteristic of the MoSi.sub.2 heating element, occurs and the polycrystalline material is pulverized, since the protective dense silica film is not formed. This is the so-called "pest" phenomenon. Under the present situation, the life of the MoSi.sub.2 heating element is, therefore, unsatisfactory for the users.
The present applicant proposed in Japanese Unexamined Patent Publication (kokai) No. 8-143,365 published Jun. 4, 1996 to form on the surface of the heating element a silicon carbide (SiC) film by CVD. The effects attained by this film are, however, not yet wholly satisfactory.